In the pollution control field, several approaches are used to remove sulfur oxides and other contaminants from a flue gas produced by the burning of a fossil fuel in order to comply with Federal and State emissions requirements. One approach involves locating and utilizing fossil fuels lower in sulfur content and/or other contaminants. A second approach involves removing or reducing the sulfur content and/or other contaminants in the fuel, prior to combustion, via mechanical and/or chemical processes. A major disadvantage to the second approach is the limited cost effectiveness of the mechanical and/or chemical processing required to achieve the mandated reduction levels of sulfur oxides and/or other contaminants.
By and large, the most widely used approaches to removing sulfur oxides and/or other contaminants from flue gas involve post-combustion clean up of the flue gas. Several methods have been developed to remove the SO2 species from flue gases.
A first method for removing SO2 from flue gas involves either mixing dry alkali material with the fuel prior to combustion, or injection of pulverized alkali material directly into the hot combustion gases to remove sulfur oxides and other contaminants via absorption or absorption followed by oxidation. Major disadvantages of this first method include: fouling of heat transfer surfaces (which then requires more frequent soot blowing of these heat transfer surfaces), low to moderate removal efficiencies, poor reagent utilization, and increased particulate loading in the combustion gases which may require additional conditioning (i.e. humidification or sulfur trioxide injection) of the gas if an electrostatic precipitator is used for downstream particulate collection.
A second method for removing SO2 from flue gas, collectively referred to as wet chemical absorption processes and also known as wet scrubbing, involves “washing” the hot flue gases with an aqueous alkaline solution or slurry in a gas-liquid contact device to remove sulfur oxides and other contaminants. Major disadvantages associated with these wet scrubbing processes include: the loss of liquid both to the atmosphere (i.e., due to saturation of the flue gas and mist carry-over) and to the sludge produced in the process; and the economics associated with the construction materials for the absorber module itself and all related auxiliary downstream equipment (i.e., primary/secondary dewatering and waste water treatment subsystems). A typical wet scrubbing system is shown in FIG. 1.
A third method, collectively referred to as spray drying chemical absorption processes and also known as dry scrubbing, involves spraying an aqueous alkaline solution or slurry which has been finely atomized via mechanical, dual-fluid or rotary type atomizers, into the hot flue gases to remove sulfur oxides and other contaminants. Major disadvantages associated with these dry scrubbing processes include: moderate to high gas-side pressure drop across the spray dryer gas inlet distribution device, and limitations on the spray down temperature (i.e., the approach to flue gas saturation temperature) required to maintain controlled operations.
There are several methods for controlling NOx emissions. Selective Catalytic Reduction (SCR) is the most common method. In these processes, ammonia is injected and mixed with the flue gas at low to medium temperatures. The mixture then flows across a catalyst (often vanadium based over a stainless steel substrate) and the Nx is reduced to N2. The problems with SCR systems is the high initial cost, high cost of ammonia which is thermally or chemically decomposed, and the introduction of ammonia into the gas stream causing problems with the formation of ammonium bisulfate and ammonia slip the atmosphere. Selective Non-catalytic Reduction (SNCR) methods are also employed. In these processes ammonia or urea in injected into hot flue gases resulting with a direct reaction forming N2. The problems with SNCR systems is the challenges with mixing and maintaining prober residence time and operating conditions for the reactions to take place optimally, sensitivity to changes in operating load, the high cost of ammonia which is thermally or chemically decomposed (even more than SCRs), and the introduction of ammonia into the gas stream causing problems with the formation of ammonium bisulfate and ammonia slip (as high as 50 ppm or higher) to the atmosphere.
NOx removal through injection of sodium bicarbonate (NaHCO3) has been demonstrated by NaTec and others.
In the prior art for wet chemical NOx reduction, the use of oxidants such as hydrogen peroxide is employed. Hydrogen peroxide is an oxidizing agent for organic and inorganic chemical processing as well as semi-conductor, applications bleach for textiles and pulp, and a treatment for municipal and industrial waste. Hydrogen Peroxide (H2O2) is an effective means of scrubbing Nitrogen Oxides. It has been used for many years. The use of H2O2 and HNO3 to scrub both NO and NO2 is an attractive option because the combination handles widely varying rates of NO to NO2, adds no contaminants to the scrubbing solution or blow-down/waste stream and allows a commercial product to be recovered from the process, i.e. nitric acid or ammonium nitrate.
Gas scrubbing is another common form of NOx treatment, with sodium hydroxide being the conventional scrubbing medium. However, the absorbed NOx is converted to nitrite and nitrate which may present wastewater disposal problems. Scrubbing solutions containing hydrogen peroxide are also effective at removing NOx, and can afford benefits not available with NaOH. For example, H2O2 adds no contaminants to the scrubbing solution and so allows commercial products to be recovered from the process, e.g., nitric acid. In its simplest application, H2O2 and nitric acid are used to scrub both nitric oxide (NO) and nitrogen dioxide (NO2)—the chief components of NOx from many utility and industrial sources.
There are several other processes which also use hydrogen peroxide to remove NOx. The Kanto Denka process employs a scrubbing solution containing 0.2% hydrogen peroxide and 10% nitric acid while the Nikon process uses a 10% sodium hydroxide solution containing 3.5% hydrogen peroxide. A fourth process, the Ozawa process, scrubs NOx by spraying a hydrogen peroxide solution into the exhaust gas stream. The liquid is then separated from the gas stream, and the nitric acid formed is neutralized with potassium hydroxide. The excess potassium nitrate is crystallized out, and the solution reused after recharging with hydrogen. In addition to the methods cited above in which NOx is oxidized to nitric acid or nitrate salts, a series of Japanese patents describe processes and equipment for reducing NOx to nitrogen using hydrogen peroxide and ammonia.
Also worth mentioning is the fact that H2O2 is used for the measurement of Nitrogen Oxide in the Standard Reference Method 7 of the Code of Federal Regulations (CFR) promulgated test methods published in the Federal Register as final rules by the US Environmental Protection Agency (EPA). In this procedure, an H2O2 solution is used in a flask to effectively capture the NOx. This, however is a slow reaction that requires several hours to complete.
There are two primary reasons that H2O2 has not gained widespread use as a reagent for removal of NOx in utility and large industrial applications. The first is that it is not a selective oxidant. Most of these sources also contain other species, primarily, SO2 which are also effectively removed with hydrogen peroxide. Thus, a large quantity of H2O2 would be required compared to the amount of NOx removal sought. Even after a limestone scrubber, the amount of SO2 present in flue gas may be equal to or greater than the amount of NOx.
The second reason that H2O2 has not gained widespread use is the cost, especially when much more is required due to reactions with SO2, for example, which can be better done prior to the H2O2 stage.
The overall reactions are:3H2O2+2NO→2HNO3+2H2O  1)H2O2+2NO2→2HNO3  2)H2O2+SO2→H2SO4  3)
Oxidation utilizing gases have been demonstrated in the art. It has been shown that over 90% of gas phase NO can be converted to NO2 rapidly by ClO2 at an applied rate of approximately 1.2 kg ClO2/kg NO. This of course requires proper mixing conditions. ClO2 is a much stronger oxidizer than hydrogen peroxide, sodium chlorate or sodium chlorite and would be a preferred oxidizer. Ozone is also a possibility, but has orders of magnitude greater capital costs relative to ClO2 generators.
Sulfur dioxide reacts with chlorine dioxide in the gas phase to form sulfuric and hydrochloric acid.2ClO2+5SO2+6H2O→5H2SO4+2HCl  4)
Assuming SO2 is the dominant species in the ClO2 reaction in the presence of SO2 and NO, then it is advisable, according to this invention, to add ClO2 after having scrubbed out SOx to keep the economics of adding ClO2 good.
A different process employs a proprietary oxidizing compound plus dilute sulfuric acid in a first stage and an irreversible process involving proprietary solutions and chemistries in a second stage. The system operates at greater than 99% efficiency on both NO and NO2 and will accommodate ambient temperature gas streams.
The prior art also does not teach simultaneous removal of mercury and NOx, especially elemental mercury (Hg°) removal. The prior art does teach limited capture of mercury using activated carbon and capture of oxidized mercury (Hg+2 such as in the form of HgCl2) (U.S. Pat. No. 6,503,470 to Nolan, et al.) in wet scrubbers that use an alkali reagent. This process also uses additives such as sodium hydrogen sulfide (NaHS) or other sulfides to chemically bind with the mercury to form compounds such as HgS.